Method for the production of bonded abrasive disc wheels

ABSTRACT

A method is provided for producing bonded abrasive disc wheels with surface discontinuities therein having a uniform density throughout. The surface discontinuities are produced on the upper surface of an abrasive mixture by projecting means from beneath the mixture and then inverting the mixture.

United States Patent lnventor Charles V. Rue

Tltfin, Ohio App]. No. 790.745

Filed Jan. 13. I969 Patented Nov. 23, I97I Assignee International Telephone and Telegraph Corporation New York, N.Y.

METHOD FOR THE PRODUCTION OF BONDED ABRASIVE DISC WHEELS 4 Claims, 8 Drawing Figs.

U.S. Cl 51/295, 51/298, 51/308, 5 H309 Int. Cl B24!) 1/00 Field of Search 51/295,

I 56 References Cited UNITED STATES PATENTS 299,055 5/1884 (3611166 51/295 2226607 12/1940 Gilmore... 51/298 3.191.876 6/1965 Bidwell 51/298 Primary Examiner-Donald J. Arnold Attorneys-C. Cornell Remsen, Jr., Walter J. Baum, Percy P. Lantzy, J. Warren Whitesel, Delbert P. Warner and James B. Raden ABSTRACT: A method is provided for producing bonded abrasive disc wheels with surface discontinuities therein having a uniform density throughout. The surface discontinuities are produced on the upper surface of an abrasive mixture by projecting means from beneath the mixture and then inverting the mixture.

PATENTEDuuv 23 l97l 3,622,288

sum 2 BF 2 METHOD FOR THE PRODUCTKON F BONDED ABRASIVE DISC WHEELS This invention relates to new and improved methods for the production of grinding discs. More particularly, it relates to the production of bonded abrasive disc wheels.

In the production of grinding discs, it has been found to be advantageous to introduce surface discontinuities such as perforations, slots and the like into the surface of the discs. These surface discontinuities serve several purposes which enhance the effectiveness of the grinding discs and, also, tend to extend the useful life of the discs. For example, the use of perfora tions, slots or other surface discontinuities produces a desirable interrupted cut, increases the unit pressure which can be applied to the workpiece, provides coolant channels and, also, provides clearance space for filing dust, chips and the like.

Generally, these discs are constructed with a plurality of perforations uniformly spaced over the grinding surface. Usually, the perforations are circular in shape. The axis of the perforations are normally perpendicular to the broad surface of the disc and parallel to the axis of rotation.

Heretofore, a standard manufacturing procedure generally has been employed for constructing these grinding discs. A pattern of pin locations is laid out on a steel backplate (hereinafter tenned the pinholder and upstanding steel pins are fixed to this plate. The pins are of the desired shape and diameter, usually about 1/2-] inch, and are slightly longer than desired in the final pressed disc. The pinholder and a second plate, hereinafter termed the stripper plate," are drilled on the same layout pattern. The stripper plate is then fitted over the pins projecting from the pinholder. Both the pinholder and stripper plate are of the same diameter as the desired final disc diameter, and they closely fit into a mold cavity or matrix into which the mixture of abrasive and bonding agents is placed for leveling and pressing. The pin set consisting of the pins, the pinholder, and the stripper plate is then pressed directly down into the mold forcing the pins down through the mixture of abrasive and bonding agents. After the disc has been pressed to its final thickness, the stripper plate is held down, and the pinholder with its complement of pins is pulled out. Finally, the pressed grinding disc and the stripper plate are ejected from the mold cavity, and the finished grinding disc is separated manually from the plate.

The above-described standard manufacturing procedure has many advantageous features. However, a serious problem has been encountered when this standard method has been employed for producing bonded abrasive disc wheels. For best grinding results, a grinding disc should exhibit a uniform density from its face (top surface as pressed) to its back (bottom surface as pressed). But, inherent in the standard manufacturing procedure employed heretofore has been the problem of achieving this uniformity.

Due to the nature of the constituents of the abrasive mixture, the pressure exerted as the pins are forced down into the mixture causes the mixture to compact in the area below the pins. The mixture does not spread out and distribute uniformly throughout the mold cavity. Thus, the density of the mixture is substantially greater where compacted at the bottom of the mold than at the top of the mold. Therefore, the discs produced by this standard method tend to have a gradation of density from their faces to their backs. Hence, as these discs are worn down in normal usage, there is a tendency for the discs to grind harder" and burn or injure the piece being ground unless frequent heat checks are performed and adjustments are made to reduce the grinding pressure being applied. It will be recognized that such procedures are time consuming and costly, but they have been necessary in order to prevent damaged work products.

Therefore, it is an object of the present invention to provide a new and improved method for producing grinding discs. The discs produced by this method have all the advantages of standard perforated discs and, also, have uniform grinding action from initial use to the wear-out point.

Another object is to provide a method for producing bonded abrasive disc wheels with surface discontinuities such as perforations, slots, and the like, having uniform density throughout the body of each of said disc wheels.

A further object is to provide a new and improved method for producing uniform density, perforated bonded abrasive grinding discs which are more effective, efficient, and economical than grinding discs produced heretofore.

According to one aspect of the invention, an abrasive mixture is introduced into a mold assembly. A means is provided for introducing a plurality of surface discontinuities such as perforations, slots, holes, cavities and the like into the abrasive mixture from a point beneath the mold assembly. As this means is projected upwardly from a point beneath the mold assembly and into the abrasive mixture in the mold cavity, the mixture uniformly distributes itself about the projecting means responsive to a gravity flow of the abrasive mixture. Then, the

entire assembly is inverted and the means for introducing the surface discontinuities is removed by raising it above the abrasive mixture. Thus, a plurality of surface discontinuities are left in the abrasive mixture. These surface discontinuities are then filled with a secondary mix material, and pressure is applied to the surface of the abrasive mixture to press it into a disc having the desired thickness. As a result, the abrasive mixture has a uniform density throughout and, consequently, the final abrasive disc has the desired uniformity without any gradations in density from the face to the back.

The above-mentioned and other features and object of this invention and the manner of obtaining them will become more apparent, and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, in which:

FIG. I is a perspective view of a molding apparatus for carrying out a method of the present invention with the front portion of the mold assembly broken away to enable one to view the interior of the mold assembly;

FIG. 2 is a cross-sectional view of the apparatus as seen along line 2-2 of FIG. 1 after the abrasive mixture has been loaded into the mold;

FIG. 3 is a cross-sectional view of the apparatus as seen along line 2-2 of FIG. 1 after the stripper plate, the mold assembly and the abrasive mixture have been lowered to the pinholder and the pressplate has been placed in the top of the mold;

FIG. 4 is a cross-sectional view of the apparatus as seen along line 2-2 of FIG. 1 after the entire assembly has been inverted and press pins have been inserted through holes in the pinholder;

FIG. 5 is a cross-sectional view of the apparatus as seen along line 2-2 of FIG. 1 after the pinholder and pins have been removed and the stripper plate has been removed;

FIG. 6 is a cross-sectional view of the apparatus as seen along line 2-2 of FIG. 1 after the cavities in the abrasive mix have been filled with a secondary mix;

FIG. 7 is a cross-sectional view of the apparatus as seen along line 2-2 of FIG. 1 after the top pressplate has been placed in the mold; and

FIG. 8 is a top perspective view of the finished bonded abrasive disc.

Briefly, FIG. I schematically shows a mold 15 having a cavity 16, a table 12, and a pinholder assembly 11. The pinholder assembly has upstanding pins 13 thereon. The cavity 16 and the pinholder 11 have a diameter equal to the desired diameter of the end product abrasive disc. A stripper plate 14 is provided with holes 18 matching the position of the pins in the holder 11 and a diameter equal to the desired diameter of the end product abrasive disc. Hence, an abrasive material may be poured into cavity 12 and the mold 15 may be lowered so that pins 13 pass through holes 18 and into the abrasive material. Thereafter the entire assembly may be inverted, the mix slightly compacted and the pinholder l1 and stripper plate 14 removed. Then, the resulting holes are filled with a secondary soft material and pressure may be applied downwardly upon the top of the disc to compact it into its final form.

In greater detail, the pinholder plate 11 is placed on the table surface 12 with a plurality of metallic pins 13 facing upwardly from the surface of the pinholder 11. For example, a set of pins, pegs or posts having the desired shape and size mounted below the mold assembly which can be projected into the abrasive mixture in the mold assembly from below, either by raising the pins, pegs or posts up into the mixture or by lowering the mixture down into the pins, pegs or posts, is suitable. Pins, pegs or posts having a generally circular shape and a 1/2-l-inch diameter are preferred, although other shapes and sizes can be utilized. The pins 13 should be considerably longer than the length of the perforations which are desired in the final abrasive disc. The length of pins 13, selected to produce a given length of perforations in the final disc, can be determined by multiplying the perforation length desired in the final disc by the compression ratio of the abrasive mixture which is used. The abrasive mixture to be employed herein comprises an abrasive material such as aluminum oxide, silicon carbide and the like and mixtures thereof and a bonding agent. Bonding agents which may be employed herein are resinous materials, natural and synthetic rubbers and vitrified or ceramic bonding agents. When a resinous material is to be incorporated as the bonding agent herein, liquid resins, powdered resins and mixtures thereof may suitably be employed. Suitable resinous bonding agents are phenol-formaldehyde resins, polyurethane resins, epoxy resins, polyamide resins, polyimide resins and the like and mixtures thereof. Suitable vitrified or ceramic bonding agents are porcelain and glass bonding agents.

The stripper plate 14 is positioned over the pins 13 and the mold assembly 15, having a recess 16, is positioned over the stripper plate M in a manner such that the recess 16 is in alignment with the stripper plate 14. The mold assembly 15 is latch pinned to the stripper plate 14 by a suitable latch means 17. The attached stripper plate 14 and mold assembly 15 can be raised as a unit to the top of the pins 13 to facilitate loading of an abrasive mixture into the recess 16 of the mold assembly 15.

After an abrasive mixture 21 has been ii'itroduced into the recess 16 of the mold assembly 15 as best illustrated in FIG. 2, the mixture 21 is mixed by suitable means to insure a homogeneous mixture. Then the mixture 21 is leveled by suitable means such as raking.

As illustrated in FIG. 3, after the abrasive mixture 21 has been loaded into the recess 16 of the mold assembly 15 and mixed and leveled, the stripper plate 14 and the mold assembly 15 are lowered as a unit to the pinholder surface 11. The pins 13 now project through or nearly through the uniformly distributed abrasive mixture 21 depending upon the length of the pins 13 to be employed. As the means for introducing the surface discontinuities is projected into the abrasive mixture in the mold cavity from a point beneath the mold assembly, the mixture uniformly distributes about the projecting means by gravity flow of the abrasive mixture. Thus, the abrasive mixture will then have a uniform density throughout and, consequently, the final abrasive disc will have the desired uniformity without any gradations in density from the face to the back.

The latch means 17 is then retracted. A bottom pressplate 22 is inserted into the top of recess 16. The bottom pressplate 22 may have nuts or studs attached thereto if desired for molding into the back of the disc. However, as illustrated in FIG. 3, the plate 22 employed herein is flat.

The entire assembly is inverted by any convenient means. The bottom pressplate 22 is now on the bottom (FIG. 4) of the assembly. Short press pins 23 are then inserted into holes 24 in pinholder 11 so that the stripper plate 14 can be pressed slightly to compact the abrasive mixture 21. After this slight compression, the latch means 17 is engaged and the pins 13 and pinholder 11 are removed from the assembly leaving a plurality of holes or cavities 25 in the abrasive mixture 21 as ilustrated in FIG. 5. The latch means 17 is then retracted and the stripper plate 14 is removed from recess 16.

The plurality of holes or cavities 25 in the abrasive mixture 21 is then filled with a suitable secondary mix material 26 (FIG. 6). The secondary mix material 26 should be a relatively soft material, much softer than the abrasive mixture 21. Hollow ceramic beads were employed herein as the secondary mix material 26. Other suitable materials which can be employed for this purpose are glass beads, bloated clay spheres, plastic beads, plastic bubbles and the like and mixtures of these materials.

Once the holes or cavities 25 have been filled, excess secondary mix 26 is removed by any convenient means such as air blasting. Then, a top pressplate 27 is positioned in recess 16 as illustrated in FIG. 7. Heavy pressure is applied to this top pressplate in order to press the abrasive disc to the desired thickness.

Finally, the disc is removed from the mold assembly. First, the top pressplate 27 is removed. Then the mold assembly 15 is held down and pressure is applied to the bottom pressplate 22 to push the abrasive disc upwardly and out of the mold assembly 15. After removal from the mold assembly 15, the abrasive disc (FIG. 8) can be further treated if desired, for example, by curing or firing, to produce the final bonded abrasive disc. Also, the secondary mix which was added to fill the surface discontinuities in order to prevent these holes or cavities from collapsing during processing may be removed at this point or may be retained in the finished disc.

While the principles of the invention have been described above in connection with specific apparatus and applications, it is to be understood that this description is made only by way of example and not as a limitation on the scope of the invention.

I claim:

l. A method for the production of bonded abrasive disc wheels with surface discontinuities therein having a uniform density throughout, said method comprising: forming an abrasive mixture including an abrasive material selected from the group consisting of aluminum oxide, silicon carbide and mixtures thereof and a bonding agent selected from the group consisting of phenol-formaldehyde resins, polyurethane resins, epoxy resins, polyamide resins, polyimide resins, natural and synthetic rubbers, vitrified bonds and mixtures thereof, introducing said abrasive mixture into a mold assembly, providing a means for introducing a plurality of surface discontinuities into said abrasive mixture in said mold assembly from a point beneath said mold assembly, projecting said means into said abrasive mixture to produce said surface discontinuities, inverting the entire assembly, removing said means for introducing surface discontinuities from said abrasive mixture, leaving a plurality of surface discontinuities in said abrasive mixture, filling said surface discontinuities with a soft secondary mix material selected from the group consisting of ceramic beads, bloated clay spheres, plastic beads, plastic bubbles and mixtures thereof, said secondary mix material being substantially softer than said abrasive mixture and being of a size smaller than the diameter and depth of said surface discontinuities, applying pressure to the surface of said abrasive mixture to press said abrasive mixture into a disc having the desired thickness and removing said abrasive disc from said mold assembly.

2. The method of claim 1 wherein the means for introducing surface discontinuities is a plurality of pins.

3. The method of claim 2 wherein the pins are mounted on a pinholder and are projected into the abrasive mixture through a stripper plate, said stripper plate being aligned with the mold assembly and the pinholder so that the pins project properly into the abrasive mixture from a point beneath the mold assembly.

4. The method of claim 1 wherein said ceramic beads are glass beads. 

2. The method of claim 1 wherein the means for introducing surface discontinuities is a plurality of pins.
 3. The method of claim 2 wherein the pins are mounted on a pinholder and are projected into the abrasive mixture through a stripper plate, said stripper plate being aligned with the mold assembly and the pinholder so that the pins project properly into the abrasive mixture from a point beneath the mold assembly.
 4. The method of claim 1 wherein said ceramic beads are glass beads. 